A very simple script for mpv that allows playing YouTube audio while displaying the videos thumbnail.

mpv-yt-thumbnail-as-cover-art.lua

--Very simple mpv script that disables video on YouTube videos
--and instead shows the video thumbnail.
--Because there is no toggle for this I recommend only calling
--the script via the "--scripts-append" flag unless you always
--want to just see video thumbnails.

--Set this to 1 if you use Windows
windows = 0

function thumbnail_as_cover()
	--check if the video is from YouTube
	path = mp.get_property("path")
	is_yt = string.find(path, "www.youtube.com")
	if ( is_yt == nil )
	then
		return
	else
		print("YouTube video detected! Using thumbnail as cover!")
		--generate a url to the thumbnail file
		vid_id = mp.get_property("filename")
		vid_id = string.gsub(vid_id, "watch%?v=", "") -- Strip possible prefix.
		vid_id = string.sub(vid_id, 1, 11) -- Strip possible suffix.
		thumb_url_maxres1 = string.format("https://i.ytimg.com/vi_webp/%s/maxresdefault.webp", vid_id)
		thumb_url_maxres2 = string.format("https://i.ytimg.com/vi/%s/maxresdefault.jpg", vid_id)
		thumb_url_sd = string.format("https://i.ytimg.com/vi_webp/%s/sddefault.webp", vid_id)
		thumb_url_hq = string.format("https://i.ytimg.com/vi_webp/%s/hqdefault.webp", vid_id)

		--ensuring we only grab the audio
		mp.set_property("ytdl-format", "bestaudio")

		if ( windows == 1 )
		then
			--set the yt thumbnail as cover art (Windows)
			thumb_urls = thumb_url_maxres1 .. ";" .. thumb_url_maxres2 .. ";" .. thumb_url_sd .. ";" .. thumb_url_hq
			mp.set_property("cover-art-files", thumb_urls)
		else
			--set the yt thumbnail as cover art (Linux)
			thumb_urls = thumb_url_maxres1 .. ":" .. thumb_url_maxres2 .. ":" .. thumb_url_sd .. ":" .. thumb_url_hq
			thumb_urls = string.gsub(thumb_urls, "://", [[\://]])
			mp.set_property("cover-art-files", thumb_urls)
		end
	end
end

--start on every new file in a playlist
mp.register_event("start-file", thumbnail_as_cover)

If you get some 404 not found errors that's totally fine. Just means that the video you're trying to see doesn't have any high res thumbnails available and thus skips to the next best option.

Here is a template for detecting os:

local platform = nil --set to 'linux', 'windows' or 'macos' to override automatic assign

if not platform then
  local o = {}
  if mp.get_property_native('options/vo-mmcss-profile', o) ~= o then
    platform = 'windows'
  elseif mp.get_property_native('options/input-app-events', o) ~= o then
    platform = 'macos'
  else
    platform = 'linux'
  end
end